Secondary ion mass spectroscopy

ABSTRACT

In a secondary ion mass spectroscopic instrument including a source of ions to be measured and a quadrupole mass analyzer, a baffle is disposed in the instrument between the source and the analyzer so as to block direct line-of-sight communication therebetween and thereby prevent fast neutral particles, photons and fast electrons and reflected fast ions from reaching the analyzer.

United States Patent [1 1 [111 3,859,226

Schillalies Jan. 7, 1975 l SECONDARY ION MASS SPECTROSCOPY PrimarExaminer-Archie R. Borchelt 751 tzHl tShlll,Rdf, Y 1 men or z gg I aleson or Assistant ExaminerC. E. Church Attorney, Agent, or Firm-Spencer &Kaye [73] Assignee: Leybold-Heraeus-Verwaltung G.m.b.I-l.,Koln(Bayental), Germany 22 Filed: Nov. 12,1973 [57] ABSTRACT [21] Appl.No.: 414,768 In a secondary ion mass spectroscopic instrument includinga source of ions to be measured and a quadrupole mass analyzer, a baffleis disposed in the instru- [301 Forelgn Apphcatmn Pnomy Data mentbetween the source and the analyzer so as to NOV. ll, 1972 Germany2255302 block direct i f g communication therebe tween and therebyprevent fast neutral particles, pho- 250/282 j gggz tons and fastelectrons and reflected fast ions from o a t i h l 58 Field of Search250/309, 307,282, 292 mg e m yzer [56] References Cited 8 Claims, 2Drawing Figures UNITED STATES PATENTS 3,501,630 3/1970 Brubaker 250/2921 SECONDARY ION MASS SPECTROSCOPY BACKGROUND OF THE INVENTION cles,photons and fast electrons and reflected fast ions.

This can be eliminated by arranging the secondary electron multiplierwhich is disposed at the output of the quadrupole analyzer, to beaxially offset or rotated by 90 and by deflecting the ions leaving thequadrupole analyzer with the aid of a suitable deflecting field.

Such devices, however, are not completely satisfactory, since, on theone hand, further tertiary particles can be produced at the input of thesecondary electron multiplier and, on the other hand, a relatively largeportion of the ions to be detected can be lost if, as often occurs, thedeflecting field is not accurately aligned. Finally the devices forproducing the deflecting field are mechanically complicated and thusexpensive.

SUMMARY OF THE INVENTION It is therefore the object of the presentinvention to provide a device for secondary ion mass spectroscopy,including a probe from which the ions emanate, a qaudrupole massanalyzer for the ions and a secondary electron multiplier forregistering the ions, which does not exhibit the above-describeddrawbacks.

These and other objects according to the present invention are achievedby disposing a baffle, which prevents direct line-of-sightcommunication, in such device between the probe and the entrance planeof the quadrupole mass analyzer. Such a baffle could be formed as aplane circular metal foil and prevents interfering particles of the typementioned above from entering directly into the quadrupole mass analyzerand passing therethrough. The danger of the production of annoyingtertiary particles in the region of the exit plane of the quadrupole isalso eliminated so that the secondary electron multiplier need no longerbe axially offset or rotated. The comlicated devices for producing thedraw field can also be eliminated.

It is particularly advantageous to arrange the baffle within anelectrostatic lens which is disposed between the probe and thequadrupole mass analyzer. Such an electrostatic lens can be so designedthat the ions travel around the baffle, so that the intensity loss ofthe ion stream due to the baffle will be low.

In an advantageous embodiment of apparatus according to the invention,the lens is composed of three tubes which form two tubular lenses. Thebaffle is here arranged in the center tube and is connected thereto tobe at the same electrical potential as the center tube. Theelectrostatic properties of such a lens can be selected so that particlepaths in the center portion of the lens extend parallel to the axis. Thebaffle in the center tube does not create any interference with thepotential distribution with the lens since the baffle is at the samepotential as the center tube. The significant advantage of such a lensis that a characteristic analagous to that producing a strong chromaticcolor error in an optical lens will automatically result, particularlywith a focal length which is selected to be relatively short. Thisproperty can be utilized to effect a rough energy analysis of theparticles.

The chromatic aberration effects of a lens can be rep resented by thefollowing equation:

where 5; is the diameter of a beam at the focal point of the lens, a isthe convergence angle of the beam toward the focal point, E is the meanenergy of the beam, AE is the beam energy deviation range, and Cp is thechromatic aberration constant of the lens. Those particles whichpresentlarge convergence angles a produce a large spot with a diameter8p. Thus, short focal lengths are preferred for producing largechromatic aberrations. Since the axial portion of the beam is blocked,those particles with a large AE will not reach the quadrupole entrance.

Since the ions to be analyzed in secondary ion mass spectroscopygenerally have an intensity maximum between 2 and l0eV with an intensitydistribution which tapers toward higher energies, the higher energyparticles often impede the analysis of those masses which have a lowerenergy value directly adjacent to that of masses with high energy. Theabove-described rough energy analysis effected by the lens connectedahead of the quadrupole prevents the occurrence of these undesiredeffects.

Finally, the tubular lenses permit accurate adaptation of the lens tothe acceptance characteristics of the quadrupole so that the device canbe operated in its best possible manner. Within the scope of thisrequirement it is finally advantageous for the probe to be disposed inthe focal plane of the tubular lens facing the probe and for the focalplane of the second lens to lie in the entrance plane of the quadrupolemass analyzer.

A further advantageous solution of the problem of the present inventioninvolves disposing the bafile within an energy analyzer located betweenthe probe and the quadrupole mass analyzer. The energy analyzer may be aknown analyzer which is suited for focusing decelerated particles. Sucha combination of a known energy analyzer and a quadrupole mass analyzerhas been found to be particularly suitable. Between the energy analyzerand the mass analyzer there need merely be provided a deceleratingdevice for the ions. This decelerating device may be constituted, forexample, by a grid or an electrostatic lens. A lens connectedtherebetween also permits adaptation of the energy levels and acceptancecharacteristics of the energy analyzer to the quadrupole system. Thecutting out of the center beam is then assured already by the baffledisposed in the analyzer.

A baffle according to the invention can be inserted in any known massspectroscopic apparatus employing a quadrupole mass analyzer.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic cross-sectionalview of one pre-.

ferred embodiment of the invention.

FIG. 2 is a view similar to that of FIG. 1 of a second preferredembodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the basic elementsof apparatus according to the invention which includes a probe, orsample, from which the ions to be analyzed emanate. A known quadrupolemass analyzer 2 is provided for the mass analysis of these ions. Suchanalyzers are known to be composed of four parallel cylindrical rods,pairs of which are provided with superposed alternating and constantvoltages which filter out all ions except those of a predetermined mass.The measurement of the ions passing through the quadrupole 2 is effectedwith the aid of a known secondary electron multiplier 3.

Between the probe 1 and the quadrupole 2 there is disposed anelectrostatic lens 7 composed of three tubes 4, and 6. In the center 5 abaffle 9 is mounted through the intermediary bars 8 to prevent a directline-of-sight communication between probe 1 and the entrance plane ofthe quadrupole 2. I

The electrostatic focuss ing properties of the tubular lenses 10 and 11formed by tubes 4, 5 and 6 are selected so that the probe 1 lies in thefocal plane of lens 10, the entrance plane of quadrupole 2 lies in thefocal plane of lens 11, and the particle paths 12 in the region of thecenter tube 5 extend parallel to the lens axis 13. Thus lens 7 issimultaneously suited for producing a rough energy analysis. The ionspenetrating the quadrupole 2 are measured, or detected, by the secondaryelectron multiplier. The signals produced thereby are processed by theelectronic system shown as block 14, which can be of any well known typecurrently used for this purpose and which, therefore, will not bedescribed in detail.

In FIG. 2, an energy analyzer 15 is additionally disposed between lens 7and probe 1. Such an arrangement is known and is described, for example,in the periodical Journal of Applied Physics, Volume 43, No. 5, at page2291 (New York).

The baffle 9' in this embodiment is mounted inside analyzer 15 in amanner not illustrated in detail. Thus a line-of-sight communicationbetween probe 1 and the secondary electron multiplier 3 is avoided.

The energy analyzer 15 includes deceleration grids l6 and a draw grid 17producing the requisite acceleration field, with the aid of which theenergy of the ions passing around baffle 9 is analyzed.

In order to decelerate the ions leaving the energy analyzer 15 to theentrance speed suitable for the quadrupole 2, a suitable decelerationunit is provided. In the illustrated embodiment this is lens 7 of FIG.1, with baffle 9 removed. This lens may be replaced, however, by a knowndeceleration grid or any like known element disposed in the exit regionof the energy analyzer 15.

The arrangement shown in FIG. 2 has been found to be very advantageoussince it permits the realization of an improvement in the energyresolution without interfering with optimum adaptation to the inputrequirements of the quadrupole 2.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

I claim:

1. In a secondary ion mass spectroscopy apparatus composed of a probefrom which the ions to be measured emanate, a quadrupole mass analyzerpresenting an entrance plane for the ions, and a secondary electronmultiplier for receiving the ions to be measured, the improvementcomprising baffle means disposed between said probe and said entranceplane of said quadrupole mass analyzer for blocking direct line-of-sightcommunication therebetween.

2. An arrangement as defined in claim I wherein the apparatus furtherincludes an electrostatic lens disposed between said probe and saidquadrupole mass analyzer, and said baffle means is located in said lens.

3. An arrangement as defined in claim 2 wherein said lens comprisesthree successive tubes which define two successive tubular lenses, andsaid baffle means is disposed in the center one of said tube and isplaced at the same potential as said center tube.

4. An arrangement as defined in claim 3 wherein said probe has an ionemission surface disposed in the focal plane of that one of said tubularlenses which is nearer said probe and the focal plane of the other saidlens lies in said entrance plane of said quadrupole mass analyzer.

S. An arrangement as defined in claim 3 wherein said lenses are givenfocal lengths which are sufficiently short to produce energy-dependentdeflections.

6. An arrangement as defined in claim 1 wherein the apparatus furtherincludes an energy analyzer disposed between said probe and saidquadrupole mass analyzer, and said baffle means is disposed in saidenergy analyzer.

7. An arrangement as defined in claim 6 wherein said energy analyzerconstitutes means for focusing slow particles.

8. An arrangement as defined in claim 7 further comprising iondeceleration means disposed between said energy analyzer and said massanalyzer.

1. In a secondary ion mass spectroscopy apparatus composed of a probefrom which the ions to be measured emanate, a quadrupole mass analyzerpresenting an entrance plane for the ions, and a secondary electronmultiplier for receiving the ions to be measured, the improvementcomprising baffle means disposed between said probe and said entranceplane of said quadrupole mass analyzer for blocking direct line-of-sightcommunication therebetween.
 2. An arrangement as defined in claim 1wherein the apparatus further includes an electrostatic lens disposedbetween said probe and said quadrupole mass analyzer, and said bafflemeans is located in said lens.
 3. An arrangement as defined in claim 2wherein said lens comprises three successive tubes which define twosuccessive tubular lenses, and said baffle means is disposed in thecenter one of said tube and is placed at the same potential as saidcenter tube.
 4. An arrangement as defined in claim 3 wherein said probehas an ion emission surface disposed in the focal plane of that one ofsaid tubular lenses which is nearer said probe and the focal plane ofthe other said lens lies in said Entrance plane of said quadrupole massanalyzer.
 5. An arrangement as defined in claim 3 wherein said lensesare given focal lengths which are sufficiently short to produceenergy-dependent deflections.
 6. An arrangement as defined in claim 1wherein the apparatus further includes an energy analyzer disposedbetween said probe and said quadrupole mass analyzer, and said bafflemeans is disposed in said energy analyzer.
 7. An arrangement as definedin claim 6 wherein said energy analyzer constitutes means for focusingslow particles.
 8. An arrangement as defined in claim 7 furthercomprising ion deceleration means disposed between said energy analyzerand said mass analyzer.